Echo-planar imaging (EPI) using blood oxygenation dependent (BOLD) contrast is widely used for functional MRI (fMRI) in Neuroscience and Clinical research applications. Although EPI is capable of sampling the time course of the hemodynamic response with a standard temporal resolution of 2-3 seconds for whole brain mapping and with correspondingly faster temporal resolution for partial brain coverage, there is increasing interest in achieving order of magnitude faster sampling rates for whole brain mapping to resolve heartbeat-related physiological signal fluctuation to increase sensitivity in event related fMRI, to reduce sensitivity to intra-scan head movement and to measure regional onset differences of the hemodynamic responses without resorting to jittering the task paradigm.
Recent developments of high-speed fMRI include single-shot echo-volumar imaging (EVI), Inverse Imaging (InI), highly under-sampled projection imaging (PI) and more recently multiplexed EPI or fast volumetric imaging based on single-shot 3D rosette trajectories, all of which enable temporal resolution on the order of 100 ms or less. A recent study using InI demonstrated considerable improvements in hemodynamic response estimation using a moving average filter to suppress physiological noise.
Echo-volumar imaging (EVI), one of the first 3D single-shot imaging techniques, was included in the first description of EPI. The method has been challenged by the inability of whole body gradient systems to encode 3D k-space sufficiently rapidly, resulting in geometrical image distortion, signal dropouts and spatially varying blurring of the point spread function due to magnetic field inhomogeneity and transverse signal relaxation. Using the improved gradient performance afforded by a dedicated head gradient system, EVI was demonstrated with a 64×32×7 matrix and 3.8 mm×6.3 mm×5 mm spatial resolution with a readout duration of 70 ms. Using local excitation to achieve partial brain coverage, a 64×64×10 matrix with 3.75 mm×5 mm×5 mm voxel size was demonstrated.
After this initial phase of feasibility studies in the 1990s using 1.5 T scanners, there has been renewed interest in recent years. An improved version of EVI has been introduced that uses reduced field of view (FOV) encoding, outer volume suppression and a surface coil at 3 Tesla. Integration of parallel imaging has led to considerable improvement in image quality and proof-of-concept at 7 Tesla. A variant of EVI using a square spiral with 14×14×14 spatial matrix and 14 mm voxel dimensions enables detection of the negative dip across the brain with 100 ms temporal resolution.
Although increasing the temporal resolution of fMRI is the principal goal, the increased efficiency (SNR per unit time) of 3D versus 2D encoding makes EVI attractive. EVI is also considerably less sensitive to physiological noise than segmented 3D EPI methods, which are affected by signal fluctuations between segments that lead to ghosting and increase apparent physiological signal fluctuation.
Despite the technical advances, the need for specialized hardware, persistent image quality constraints due to geometrical image distortion, blurring and signal drop outs that are exacerbated by head movement, as well as signal drifts due to gradient instability and steady-state effects remain considerable challenges for routine applications, in particular at high magnetic field strength. Practical applications of EVI are also hampered by time-consuming image reconstruction of large amount of data generated by EVI such that there is a need for real-time fMRI with EVI. The present invention satisfies these needs.